Groovy syntax

Last updated on 2024-07-23 | Edit this page

Groovy syntax

Here are some fundamental concepts of the Groovy language.

  • Single line comments (lines not meant to be executed) start with //. Multi-line comments are nested between /* and */ tags.

    GROOVY 

    // This is a single line comment. Everything after the // is ignored.
/*
    Comments can also
    span multiple
    lines.
*/

  • Variables are assigned using = and can have any value. Variables used inside a double quoted string are prefixed with a $ to denote the variable should be interpolated (replace the variable with the variable’s value). A variable is otherwise just used by it’s name.

    GROOVY 

    myvar = 1                                     // Integer
myvar = -3.1499392                            // Floating point number
myvar = false                                 // Boolean
myvar = "Hello world"                         // String
myvar = new java.util.Date()                  // Object - Abstract data structure
message = "The file $file cannot be found!"   // Variable used inside a string.
println message                               // Print variable

  • Lists (also known as arrays) are defined using the square bracket [] notation.

    GROOVY 

    emptylist = []                      // an empty list
// Lists can contain duplicates, and the values can be of any type.
mixedList = [1, 1, 'string', true, null, 5 as byte]
mixedList.add("new value")          // adds "new value" to the end of mixedList
println mixedList.size()            // prints the size
println mixedList[0]                // prints 1

  • Maps (also known as associative arrays) are defined using the [:] literal. They associate a unique string with a value, and are commonly referred to as key-value pairs.

    GROOVY 

    emptyMap = [:]                      // an empty map
mymap = [ name : "Steve", age: 43, likes: ['walks','cooking','coding']]
mymap.age = 44                      // values can be accessed using map property notation.
println mymap['name']               // alternatively values can be accessed using quoted keys.

  • Groovy supports common control structures such as if/else tests, switch/case tests, for loops, and while loops.

    GROOVY 

    // if / else test
x = false
if ( !x ) {
    return 1
} else {
    return 0
}

// switch / case test
switch (x) {
    case "found foo":
        result = "Got foo"
        // continues to following cases

    case Number:
        result = "Number"
        break          // stops at this case

    default:
        result = "default"
}

// for loop (other variants exist)
String message = ''
for (int i = 0; i < 5; i++) {
    message += 'Hi '
}

// while loop
y = 5
while ( y-- > 0 ){
    println "Are we there yet?"
}
println "We've arrived!"

  • Closures are open, anonymous, blocks of code that can take arguments, return a value and be assigned to a variable. A closure definition follows the syntax { [closureParameters ->] statements }. A closure is evaluated when it is called, returning the result of the last statement in the closure.

    GROOVY 

    // Find all elements > 1
above_one = [1, 2, 3].findAll { it -> it > 1 }

// Closure in a string
message = "The file ${ file.getName() } cannot be found!"

Groovy is very syntax-rich and supports many more operations. A full description of Groovy semantics can be found in the Groovy Documentation.

Glossary

Term Description
Cache In Nextflow, caching refers to the mechanism of reusing the results of executed processes in subsequent runs. This feature reduces computation time by avoiding the re-execution of unchanged tasks.
Channel Channels are the conduits through which data flows in a Nextflow workflow. They connect processes, allowing for the movement and transformation of data from one process to the next.
Closure In Nextflow, a closure is a block of code that can be assigned to a variable, passed as an argument, or executed.
Dataflow programming Dataflow programming is a programming paradigm that models a program as a directed graph of the data flowing between operations.
Directive Directives are annotations in a Nextflow script that provide execution instructions for processes. They control aspects like resource allocation, process retries, and container usage.
Docker Docker is an open-source platform used to automate the deployment of applications inside lightweight, portable containers that allow applications to run seamlessly in different computing environments.
Domain Specific Language A domain-specific language (DSL) is a computer language specialized to a particular application domain. Nextflow is a language for computational workflows.
DSL2 The second version of the Nextflow domain-specific language, introducing modular and reusable workflow components. DSL2 allows for more structured and maintainable pipeline development.
Executor The component responsible for managing the execution of processes in a Nextflow workflow. Executors handle task scheduling, execution, and resource allocation across different computing environments.
Groovy The programming language used to write Nextflow scripts. Groovy is a powerful, optionally typed, and dynamic language, with static-typing and static compilation capabilities. It enhances Nextflow with scripting flexibility.
Module A reusable component in a Nextflow pipeline that encapsulates a specific function or set of functions. Modules can be combined and reused across different workflows, promoting modularity and code reusability.
Nextflow A domain-specific language and toolkit for parallel and scalable computational workflows. Nextflow enables the scripting of complex data analysis pipelines in a reproducible and efficient manner.
Nextflow workflow In Nextflow, a workflow is a composition of processes and dataflow logic (i.e., channels and operators).
Operators In Nextflow, operators are methods that allow the transformation, filtering, and combination of data streams. They facilitate the management and manipulation of data within a workflow.
Parameters Variables defined in a Nextflow script or passed at runtime to customize the execution of a workflow. Parameters allow for the dynamic configuration of workflows based on input data or user-defined settings.
Pipeline A sequence of processes orchestrated by Nextflow to perform a complex data analysis task. Pipelines are defined in Nextflow scripts and can be composed of multiple workflow blocks and processes.
Process A fundamental component in Nextflow representing a computational task. Each process defines a small part of the overall workflow, encapsulating the command-line tools, scripts, and computational resources required for execution.
Profile A set of configuration options in Nextflow that can be activated to adapt the execution environment of a workflow. Profiles enable easy switching between different computing environments, like local, cloud, or cluster execution.
PublishDir A directive in a process block that specifies a directory where the output files of the process should be stored. It helps in organizing and accessing the results of workflow executions.
Script Block A block within a Nextflow workflow that contains custom script code. Script blocks allow for the embedding of standard programming constructs and commands within the flow of a workflow.
Tuple An ordered, immutable list of elements. A tuple can be seen as an ordered collection of objects of different types. These objects do not necessarily relate to each other in any way, but collectively they will have some meaning.
Workflow Block A segment of a Nextflow script defining a series of operations or processes to be executed.
WorkDir The working directory where Nextflow stores intermediate files generated by processes. It ensures reproducibility and efficient management of temporary data.

External references

Manuals

Papers